Plant cells are full of solutes, both dissolved inorganic ions and low-molecular-mass organic molecules. The concentration of solutes inside plant cells is higher than that in the growing medium, and it is much higher for the large majority of terrestrial plants. Plants expend considerable amounts of energy and resources upon acquiring or synthesising these solutes, so perhaps the first question to ask is, 'why do they do it?'
In part the reasons are historical. The salinity of the early oceans was substantially greater than it is today (Knauth, 1998). The conditions in which life evolved are still debated. It is believed that life might have been evolved in situations where freshwater diluted this salinity; however, the great majority of early life arose in the oceans. For simple physical reasons (water flows across their semipermeable membranes influenced by osmotic forces; see Chapter 3), it was necessary for cells to match the water potential of the seas to remain hydrated; so an equivalent concentration of solutes was needed. Some fundamental living processes of cells were laid down during this arcane period - long before life colonised the land. Observation has shown that these processes have been rigidly conserved; for instance the ionic requirements for protein synthesis (see Chapters 3 and 14). The ghost of the past commands the conditions that plants have to maintain in the cytoplasm of their cells today (a hundred or hundreds of mM of solutes). This is even though the concentration of salts in the growing medium may now be of the order of pM and mM, a tiny fraction of that in the present-day or ancient oceans.
In part the reasons are physical. The first challenge of life on land was to remain hydrated. As plants evolved from wetlands to dry land, the availability of water became less. Retaining water against the non-osmotic components of water potential became a priority for the first time. The soil was periodically dry, and the cells of plant roots had to retain water against the water potential of the drying soil. In addition to this, the leaves of plants were in a medium that was hardly ever saturated with water, that is the air. The moist surfaces of cells lost water to the demands of the unsaturated air - because of the vapour pressure difference. This has been the no-win situation of plant life on land. The need to acquire atmospheric carbon dioxide for photosynthetic carbon fixation meant that the cells could not be permanently waterproofed - letting in carbon dioxide meant letting out water. Cells not only had to obtain their water from drying soil, but also had to compete with the voracious demands of transpiration - some 98% of water used (see Chapter 3) - and for this they had to depend upon their own osmotic pressure.
In part the reasons are structural. Without enough water, plants and even the leaves of trees wilt. Plants still rely largely on a hydrostatic skeleton maintained by turgor pressure; that is the positive hydrostatic pressure that the cell contents exert upon the surrounding structural cell walls (see Chapter 3). Cells use the osmotic component of water potential (hence the dissolved solutes) to build the turgor pressure. Without this, leaves (or large parts of the plant in the absence of the structural thickening found in woody stems) become flaccid. Such leaves are then unable to fulfil the needs of photosynthesis and may be irreversibly damaged. The large majority of plant growth is by cell expansion. In contrast with animals, mature cells of plants contain a large central vacuole (which may be 90% or more of the volume). This is the principal way in which plants generate size, be it to get up into the light or down into the wet soil, or to expand leaves and ramify roots to capture carbon dioxide, water and nutrients. A continual increase on the quantity of solutes is needed to sustain the concentration within the growing cells, without this the turgor pressure would decrease and there would be no growth.
For all these reasons, it is a fundamental requirement for survival that plants fill their cells with solutes, whether this is in the form of inorganic ions concentrated from the growing medium or with organic solutes synthesised from sources (of principally: carbon, nitrogen, phosphorus, oxygen and hydrogen) in the atmosphere and soil.
Plants need both the major inorganic ions (for instance, potassium, magnesium and nitrates) and the numerous ions that serve the role of specific 'micronutrients'. On land these resources had to be found from an environment in which they could become rapidly depleted - in contrast to the sea, where, even at low concentrations, there was normally continual replacement. Nowadays, 'fertigation' and nutrient film techniques are common in commercial horticulture to prevent such depletion. In the soil, plants must often forage for the materials they need. Overall the flows of water and solutes are locked together in a dance of physical laws. Evapotranspiration causes a mass flow of water through the soil-plant-atmosphere system and the accumulation of salts drives localised flow of water which brings with it dissolved salts. It takes two to tango.
The solutes of plant cells and their roles are diverse. Quantitatively, the largest components are dissolved inorganic ions and low-molecular-mass organic molecules. But the term solute also includes compounds of greater molecular mass as components and products of biosynthetic and catabolic pathways and cycles, up to and including soluble proteins and nucleic acids. Not all soluble species always exist in, or are always transported in, solution. Soluble inorganic ions must often be transported anhydrously across the membrane bilayer by protein carriers. Also, there are species that are not soluble in water but are nonetheless transported throughout the plant; for instance insoluble proteins and viral particles.
The transport of solutes occurs over a large range of scale, some 10 orders of magnitude, from the order of 10 nm to cross a cell membrane to the order of 100 metres to ascend the tallest tree. The nature of the events and driving forces that underlie transport over such differences in scale are extremely different for the same solute. A potassium ion carried to the top of a tree in the xylem is in solution in water, but a potassium ion being transported across a membrane by a carrier is not in solution but is bound reversibly to a transport protein. Movement up the xylem of a tall tree is by a mass flow of solution driven largely by the evaporation of water at the leaf surface, while accumulation across a membrane is driven either directly or indirectly by energy derived from a biochemical process.
Membranes provide the compartmentalisation that is central to living processes; allowing different cells to perform different functions and allowing different processes to go on within the same cell. The concentrations of soluble metabolic intermediates of the citric acid cycle within the mitochondrion can be made relatively independent of the concentrations of the same solutes in the cell as a whole. This allows the same solute to be used for different purposes in different parts of the same cell. Extreme examples are the vacuolar compartmentalisation of malate in CAM plants (see Chapter 13) and of salts in halophytes (see Chapter 14); in both cases permitting the retention of concentrations that would destroy the cytoplasm. More generally, compartmentalisation within membrane-bound compartments provides efficiency, allowing high concentrations to exist in one place without the need for the enormous quantities that would be needed to provide the same concentration throughout the cell. The compartmentalisation of protons is universal in plant cells, with pumping out of the cytoplasm both across the plasma membrane to the outside and across the tonoplast into the vacuole. This not only provides the neutral-to-alkaline pH needed in the cytoplasm, but the electrochemical potential gradient of protons. In plant cells, it is this proton motive force (PMF) that is used both to store and couple the energy derived from biochemical processes (ATPases and py-rophosphatases, the photosynthetic and respiratory electron transport chains) with the active transport of other solutes.
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